How to Detect Circuit Board Faults?
Printed circuit boards (PCBs) are increasing in complexity and diversity. With a wide array of applications, the only requirement common to all types of PCBs is they must function in accordance with their design parameters, without errors and failures. In short, PCBs must perform flawlessly.
Complex PCBs can have hundreds of components with thousands of solder connections and that gives innumerable opportunities for failure. The Printed circuit boards manufacturing industry makes sure all their PCBs meet the above challenge of flawless working through a battery of inspection and testing procedures to ensure the quality of their products.
Assemblers detect circuit board faults before assembly through various inspection methods. After assembly is over, they employ another set of inspection and test methods to solve PCB errors.
Evolution of PCB Inspection and Test Methods
Simple circuit boards with a handful of components needed only manual visual inspection (MVI) methods to ensure solder problems and placement errors were weeded out. With increasing complexity and growing production volumes, MVI systems were found to be inadequate, as humans soon grew tired, and could not be relied upon to carry out the task of inspection repeatedly for long hours. As a consequence, inspectors missed defects and faulty boards reached later stages where it was more expensive to solve PCB errors.
This brought up the next step in inspection systems—Automated Optical Inspection (AOI) methods—now a widely accepted inline process. Assemblers effectively use AOI to inspect PCBs before and after reflow soldering to check for a variety of possible faults. Now, even pick-and-place machines incorporate AOI capabilities, allowing them to check for misalignment and faulty component placements.
With the advancement of surface mount technology, components became smaller, and this increased the board complexity along with PCBs becoming double sided and even multi-layered. Additionally, introduction of fine-pitch SMDs and BGA packages brought out the limitations of AOI, forcing assemblers to implement even better inspection methods such as the Automated X-ray Inspection (AXI) systems.
After the assembly, PCBs are often tested for in-circuit components (ICT) and for functional testing (FCT). While ICT ensures the functioning of individual components on the board, FCT offers a final go or no-go decision for the entire PCB.
Expected Faults in PCBs
Statistical data on PCBs shows the most common types of faults related to placement, soldering, and functionality. Among placement faults, components may be missing, wrong, wrong orientation, or misaligned. Among soldering faults, there may be dry or incomplete soldering, excess amounts of solder, solder bridging, and whiskers. Assemblers vary their inspection and testing methods depending on the type of defects they encounter and the effectiveness of the inspection methods.
Automated Optical Inspection
AOI methods inspect PCBs visually. The system usually employs still or video cameras to scan a well-lit board. There can be several variations, with the board being illuminated by different sources of light at various angles, and there may be more than one camera. Images from the camera are fed to into a computer, which builds a picture of the board and its contents. The memory of the computer holds the reference image of a golden board that has no faults. The computer compares the image the cameras have captured with the reference image and highlights the faults it detects.
With AOI systems, it is easy to detect faults such as open circuits, shorts, and dry solders. Moreover, it can detect missing and misaligned components. The biggest advantage with AOI is they can help solve PCB errors much better than human inspectors can, with greater accuracy, in less time, and without tiring. Therefore, manufacturers employ AOI systems inline at several points in the PCB manufacturing process.
3-D AOI systems are capable of measuring the height of components, and able to detect faults in areas that are sensitive to heights. However, they use visible light, which limits the functionality of AOI systems to line-of-sight. AOI systems are incapable of inspecting hidden connections such as under IC packages, especially BGAs.
Automated X-Ray Inspection
Chip scale packages (CSP) and Ball Grid Arrays (BGA) are special IC packages that have their connections under them. When mounted, the connections are hidden between the circuit board and the body of the IC, preventing them from being inspected visibly. Assemblers resort to AXI methods to inspect such hidden solder joints.
Printed boards are made of substrates and copper traces, and SMD components are soldered onto them. Materials usually absorb X-rays in proportion to their atomic weights. While materials containing heavier elements absorb more X-rays, those containing lighter elements allow X-rays to pass through without absorption. The PCB substrate and components are mostly made up of lighter elements, and the X-rays pass through them without being absorbed.
On the other hand, solder contains heavy elements such as indium, silver, bismuth, and tin, and these do not allow X-rays to pass through. Therefore, when inspecting a PCB assembly with X-rays, the solder joints show up with great clarity, while the traces and SMD packages are barely visible.
Therefore, AXI systems make it easy to detect and solve PCB errors such as soldering defects normally invisible to AOI systems. However, AXI systems are more expensive, and assemblers install them only if necessary.
In-Circuit Testing (ICT)
Assemblers perform testing only after completing all PCB inspections and soldering the components. For ICT, it is necessary the designer has placed testing pads at critical points in the circuit when designing the PCB layout. Usually, the designer will place the test pads on a grid so a testing jig with spring-loaded pins can connect to the pads on the PCB. This test fixture is usually called a bed-of-nails.
During ICT, the test pins can check various components for shorts, opens, resistance, capacitance, and more, for determining any errors. Usually, such bed-of-nails is specific to a circuit board, and therefore inflexible and expensive. Moreover, with circuit density increasing continuously, bed-of-nails soon reaches its limits. Assemblers use another approach instead. While a simple fixture holds the board, a single probe or a few of them move to make contact at different points as necessary. Usually, software controls the probe movements, and this makes it easy to adapt the system to different boards.
Functional Circuit Testing (FCT)
Under FCT, the circuit board assembly is powered up while test equipment is connected to simulate the actual environment the board is expected to undergo in normal use. The functional tester is unique to the board under test, and its software program sequences through various test scenarios while collecting operational data from the devices on-board.
Depending on the extent of testing, the type of inputs required, and the expected outputs from the device under test, the FCT can vary in its complexity. However, it identifies functional defects in the PCB assembly, and helps to solve PCB errors.
Assemblers use different inspection and testing methods to solve PCB errors during manufacturing and assembly. With high volumes of production and circuit complexity, the automatic visual methods have mostly replaced manual visual methods of inspection. For complex PCBs such as those using BGAs and CSPs assemblers have to use X-rays to inspect invisible solder joints.